The invention relates to machine elements and mechanisms but more particularly, the invention relates to improved power transmission wheels such as gear wheels, sprocket wheels, and pulley wheels having a plurality of teeth formed of a plastic-fiber composite material.
There are several types of power transmission wheels such as gear wheels, sprocket wheels, and pulley wheels which have a power transmission surface or layer that is formed from a composite material of plastic and embedded fibers. Some power transmission wheels are formed totally of the composite material while others may include a metallic insert which forms a hub portion and where the composite material is used in a rim portion. An example of such composite materials include fabrics impregnated with a phenolic resin. Another composite material that has the advantage of being capable of being molded by thermoplastic injection molding is a composite material of nylon reinforced with milled glass fibers having a length of 1/16 in. or less. While a thermal plastic injected nylon material containing a 40 weight percent of nylon fiber is effective for use in many power transmission wheel applications, such composite materials are not suited for high-power, long-life applications where it is customary to use materials such as cast iron and powdered metal. Power transmission wheels of cast iron are designed using material strengths from about 30,000 to 60,000 psi tensile strength. A characteristic of composites with milled glass fibers is that there are end portions of fiber juxtaposed the power transmission surface of the power transmission wheel that may lead to early failure of a component part such as a power transmission belt. In the case of phenolic composites, the power transmission surface is typically machined or molded and then worn through exposing end portions of embedded fiber.
An example of where a composite of nylon and milled glass fiber is inadequate because of component failure and where sprockets of iron are typically used is in a power transmission system of the type that uses toothed belts that engage sprockets where the belt and sprocket teeth are of the type as disclosed in U.S. Pat. No. 4,605,389 to Westof and where the belt is of the cast urethane type with an elastomer free surface construction as disclosed in U.S. Pat. No. 3,964,328 to Redmond.
Examples of various types of plastic gears and problems associated therewith are summarized in the Jan. 21, 1988 and Oct. 13, 1988 issues of Machine Design.